Proton conductivity, structural and thermal properties of $(1-x)$CsH$_{2}$PO$_{4}$–$x$Ba(H$_{2}$PO$_{4}$)$_{2}$

The structural, electrotransport, and thermodynamic properties of the $(1-x)$CsH$_{2}$PO$_{4}$–$x$Ba(H$_{2}$PO$_{4}$)$_{2}$ system in a wide range of compositions ($x$ = 0.1–0.4) were firstly studied to develop the highly conductive proton electrolytes within the medium-temperature range. At $x$ = 0–0.1, formation of disordered substitutional solid solutions, isostructural to CsH$_{2}$PO$_{4}$ ($P$2$_{1}/m$), with a decrease of the unit cell parameters and considerable increase of proton conductivity as a result of formation of vacancies in the cesium sublattice and weakening of the system of hydrogen bonds, was observed. At $x$ = 0.15–0.4, the heterophase highly conductive systems demonstrating high values of proton conductivity $\sim$10$^{-2}$ S/cm at $x$ = 0.15–0.2, stable under the long-term isothermal exposures and low humidity ($T\sim$ 200–210$^\circ$C, RH $\sim$15%), are formed. The phase transition disappears, the energy of activation of conductivity decreases from 0.9 to 0.55 eV at $x$ = 0.2. The conductivity of high-temperature phase does not vary with Ba(H$_{2}$PO$_{4}$)$_{2}$ fraction increase to $x$ = 0.2. The mechanisms of transfer of protons were discussed. It has been shown that when $x>0$.10 the contribution to proton conductivity of molecules of the water adsorbed on the phase boundary of the composite systems increases.